1332 CHAPTER 18 Sample Questions

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1 1332 CHAPTER 18 Sample Questions Couple E 0 Couple E 0 Br 2 (l) + 2e 2Br (aq) V AuCl 4 + 3e Au + 4Cl V Ag + + e Ag V Hg e 2 Hg V Fe 3+ (aq) + e Fe 2+ (aq) V Cu 2+ (aq) + 2e Cu(s) V Cd 2+ (aq) + 2e Cd(s) 0.40 V Fe e Fe V Zn e Zn 0.76 V Mn e Mn 1.18 V Al e Al V Mg e Mg V REDOX 1. What is the oxidation number of phosphorus in H 3 PO 2? In the ion H 2 P 2 O 7 2, the oxidation number for P is In which acid is the oxidation number of phosphorus lowest? H 4 P 2 O 7 H 3 PO 4 PH 3 H 3 PO 3 4. Identify the pair of compounds having underlined atoms with the same oxidation number. H 2 SO 4 and HMnO 4 HClO 3 and HNO 2 HClO 4 and H 2 Cr 2 O 7 HNO 3 and H 3 PO 4 5. The oxidation number of Cl in KClO 2 is +1 l What is the oxidation number of sulfur in S 2 O 6 2? Which statement is true for the reaction - Fe(s) + Cu 2+ (aq) Cu(s) + Fe 2+ (aq) Cu 2+ is oxidized. Cu 2+ gains in oxidation state. Cu 2+ is reduced. Fe(s) is reduced. 8. Consider the reaction, 2Fe 3+ (aq) + 2I (aq) 2Fe 2+ (aq) + I 2 (aq). Which statement is true: Fe 3+ is oxidized. Fe 3+ increases in oxidation number. Fe 3+ is reduced. I is reduced. 1

2 9. In the chemical reaction, Zn(s) + CuSO 4 (aq) ZnSO 4 (aq) + Cu(s), metallic zinc is the reducing agent. metallic zinc is reduced. copper ion is oxidized. sulfate ion is the oxidizing agent. 10. Which substance behaves as the oxidizing agent in Pb + PbO 2 + 2H 2SO 4 2PbSO 4 + 2H 2O Pb PbSO 4 PbO 2 H 2 SO In the process of oxidizing Fe 2+ to Fe 3+, Cr 2 O 7 2 is reduced to Cr 3+. How many moles of Fe 2+ are oxidized by one mole of Cr 2 O 7 2? Given the unbalanced equation, ClO 3 + NO 2 Cl + NO 3. How many moles of NO 2 will react with l mol of ClO 3? l Balance this equation, Fe 2+ + MnO 4 Fe 3+ + MnO 2 (in basic solution) The sum of the total number of moles on the left side of the balanced equation is 7 mol 6 mol 5 mol 4 mol 14. Balance the equation for this reaction carried out in a basic solution: CrO HSnO 2 HSnO 3 + CrO 2. The coefficient of HSnO 2 will be l What is the coefficient of Br when this equation is completed and balanced using the smallest integer coefficients? Br (aq) + MnO 4 (aq) Br 2 (l) + Mn 2+ (aq) (acidic solution) In acidic aqueous solution, MnO 4 reacts with Fe 2+ to give Mn 2+ and Fe 3+. Complete and balance the equation. What is the coefficient of Fe 2+ in the overall net ionic equation? l Complete and balance the equation assuming the reaction is carried out in basic, aqueous solution, MnO 4 (aq) + S 2 (aq) MnO 2 (s) + S(s). What is the coefficient for water? In acidic, aqueous solution Cr 2 O 7 2 reacts with H 2 S to give S(s) and Cr 3+ (aq). When the equation is balanced, the coefficient of the H + (aq) in the overall net ionic equation is

3 19. Balance ClO 4 + Fe 2+ ClO + + Fe 3+ in acidic solution. The coefficient of H + ions is: (E) Balance the equation for this reaction. Cr 2 O HSO 3 Cr 3+ + SO 4 2 in acidic solution. What number appears before H 2 O? (E) Balance the ionic equation for this reaction: MnO 4 + H 2 O 2 Mn 2+ + O 2 (in acidic solution). What is the coefficient for H 2 O 2? l (E) The sum of all coefficients when the equation MnO 4 + Cl Mn 2+ + Cl 2 (in acid) is balanced is: (E) 51 E values 23. What would be the E 0 value in volts for a zinc silver galvanic cell? What is the E 0 value for a cadmium silver voltaic cell? 0.6 V 2.0 V 1.2 V 0.40 V 25. What is the value of E 0 for the spontaneous reaction resulting from a suitable combination of these halfreactions? Fe 3+ + e Fe V Hg e 2 Hg V V V V V (E) 1.56 V 26. In a galvanic (voltaic) cell in which the reaction is, Cd + Cu 2+ Cu + Cd 2+, and the ions are at unit concentration (activity), the cell potential is 0.14 V 0.40 V 0.34 V 0.74 V (E) 0.37 V 27. What is E 0 for this reaction, 3Mn + 2AuCl 4 3Mn Au + 8Cl 0.18 V 2.18 V V 5.54 V 28. If E for the reaction Br 2 (l) + 2Ce 3+ (aq) 2Br (aq) + 2Ce 4+ (aq) is 0.55, what is E for Ce 4+ (aq) + e Ce V 0.04 V 0.51 V V 3

4 29. Using the following half-reactions, select the strongest reducing agent: HgO + H 2 O + 2e - Hg + 2 OH - Zn(OH) 2 + 2e - Zn + 2 OH - Ag 2 O + H 2 O + 2e - Ag + 2 OH - B(OH) H + + 9e - - BH H2 O E = V E = V E = V E = V Hg Zn Ag BH In the cell shown, which reaction occurs at the cathode? Cu(s) Cu 2+ (aq) Ag + (aq) Ag(s) Cu 2+ (aq) + 2 e - Cu(s) Ag(s) Ag + (aq) + e - Ag + (aq) + e - Ag(s) Cu(s) Cu 2+ (aq) + 2 e - (E) none of these 31. What is the E cell for a voltaic cell that operates using the half reactions below? Cu e - Cu(s) E = 0.34 V Sn e - Sn(s) E = 0.13 V 0.68 V 0.47 V V 0.21 V (E) V 32. The iodine clock reaction [S 2 O 8 2- (aq) + 2 H + (aq) + 2 I - (aq) 2 HSO 4 - (aq) + I 2 (aq)] has E = V and the reduction potential for I e - 2 I - = V. What is the reduction potential of S 2 O 8 2-? V V V V (E) V 33. Given the standard reduction potentials for the following half-reactions, Pb 2+ /Pb = V and Ni 2+ /Ni = V; which substance would be oxidized in a voltaic cell? Pb 2+ H + Ni Pb (E) Ni Which pair of substances will produce the greatest electromotive force when used as electrodes in 1 M solutions of their ions given the reactivity series? Na > Zn > Fe > Pb > H > Cu > Hg > Ag sodium and pure mercury iron and pure mercury sodium and hydrogen iron and copper (E) zinc and hydrogen 35. Using only the metals Mg, Al, Zn, Fe, Cu and Ag, together with their l M salt solutions, a voltaic cell of the highest possible voltage would be constructed using electrodes of these metals: Mg and Ag Mg and Fe Zn and Cu Al and Ag (E) Mg and Al 4

5 36. Using the potentials given at the start as well as the fact that Ce 4+ + e - Ce 3+ is V, determine which of the following reactions will be spontaneous under standard conditions. copper reduces silver ions ferrous ions reduce zinc ions bromide ions reduce zinc ions bromine oxidizes cerium(iii) ions Cells 37. In every electrolytic and galvanic (voltaic) cell the anode is that electrode at which oxidation occurs. which attracts cations. at which electrons are supplied to the solution. at which reduction occurs. 38. Which statement is true for the cell as it discharges, Zn Zn 2+ (1.0 M) Sn 2+ (1.0 M) Sn Oxidation occurs at the tin electrode. Electrons will flow from the tin electrode to the zinc electrode. The concentration of Zn 2+ will increase. The mass of the tin electrode will decrease. 39. In every electrolytic and galvanic cell, the cathode is the electrode which attracts cations which attracts anions at which oxidation occurs at which reduction occurs (E) at which electrons are removed to the solution 40. Two types of electrochemical cells, voltaic and electrolytic, can be described. Which statement about them is TRUE? (E) current flows directly from the cathode to the anode in an electrolytic cell the anode in a voltaic cell will gain weight as it runs oxidation occurs at the cathode in a voltaic cell the free energy change, ΔG, is negative for voltaic cells the potential of voltaic cells is independent of concentration Spontaneity etc 41. Which reaction is spontaneous in the direction written? 2Ag + Cu 2+ Cu + 2Ag + Fe + Zn 2+ Fe 2+ + Zn 2Al + 3Mg 2+ 2Al Mg 2Al + 3Zn 2+ 2Al Zn 42. A spontaneous electrochemical reaction has: ΔG = 0, E = 0, and K >> 1 ΔG < 0, E > 0, and K > 1 ΔG > 0, E < 0, and K < 1 ΔG > 0, E < 0, and K > 1 (E) ΔG < 0, E = 0, and K >> 1 5

6 43. Under which of the following conditions will a cell do the most work? Q/K < 1 Q/K = 1 E < 0 E = 0 (E) Q/K > Calculate K, the equilibrium constant for the reaction, Ag + (aq) + Fe 2+ (aq) Ag(s) + Fe What is the value of the equilibrium constant for the reaction between Sn 2+ and Fe(s) at 25 C given that: Sn 2+ (aq) + Fe(s) Sn(s) + Fe 2+ (aq); E = 0.30 V 1.4 x x x 10-6 (E) 7.3 x The value of E for the cell Cr(s) Cr 3+ (aq) Hg 2 2+ (aq) Hg(l) is 1.59 V. What is ΔG for the reaction at 25 C? -767 kj -460 kj -921 kj +767 kj (E) -307 kj 47. For the half-cell Pb e - Pb, E = V. What is the value of ΔG for the reaction, Pb(s) + 2 H + (aq) Pb 2+ (aq) + H 2 (g)? 12 kj kj 25 kj -25 kj (E) -12 kj 48. Calculate ΔG in kj for the reaction, Ag + (aq) + Fe 2+ (aq) Ag(s) + Fe Nernst Equation 49. Consider the equation, Cu 2+ (aq) + Fe(s) Cu(s) + Fe 2+ (aq). The standard potential for this reaction is 0.78 V. What is the potential if the concentrations are M Cu 2+ and 0.40 M Fe 2+? 0.72 V 0.75 V 0.81 V 0.84 V 50. Calculate the value of E for the cell at 25 C: Cu Cu 2+ (1.00 M) Ag + ( M) Ag A voltaic cell consists of an Mn/Mn 2+ electrode (E red = V) and a Fe/Fe 2+ electrode (E red = V). Calculate [Fe 2+ ] if [Mn 2+ ] = M and E cell = 0.78 V 0.24 M 1.1 M 1.8 M 0.80 M (E) M 52. The value of E for the reaction Pb 2+ (aq) + Zn(s) Zn 2+ (aq) + Pb(s) is 0.63 V. What is the value of E cell at 25 C when [Zn 2+ ] = M and the concentration of Pb 2+ = 1.0 M? 0.52 V 1.62 V 0.74 V 0.85 V (E) 0.41 V 6

7 53. The value of E for the reaction, Zn(s) + Cu 2+ (aq) Zn 2+ (aq) + Cu(s), is 1.10 V. What is the value of E for a cell in which [Cu 2+ ] = 1.0 x 10-5 M and [Zn 2+ ] = 1.0 M? 0.95 V 1.40 V 1.25 V 1.10 V (E) 0.80 V 54. What is the value of E for the cell, Al Al 3+ (0.500 M) Fe 3+ (0.421 M), Fe 2+ (1.0 x 10-3 M) Pt? 2.43 V 2.37 V 2.49 V 2.59 V Electrolysis 55. During electrolysis (E) positive ions gain electrons at the anode. negative ions lose electrons at the cathode. electrons enter the electrolytic cell at the cathode. ions enter the external or wired circuit and flow from plus to minus. oxidation takes place at the cathode. 56. During the electrolysis of an aqueous solution of CuSO 4 with inert electrodes (E) the anode loses mass and the cathode gains mass. the mass of the anode remains the same but the cathode gains mass. the mass of the anode decreases but the mass of the cathode remains constant. the anode and the cathode neither gain nor lose mass. both electrodes gain in mass. 57. Aqueous solutions of fluorides are difficult to oxidize at the anode of an electrolytic cell because (E) the aqueous solutions of fluorides are nonconducting. it is impossible to find the proper material from which to build the electrodes. the fluorides are not very soluble. in salts the fluoride ion has a plus charge. oxygen is released from water in preference to fluorine. 58. In the electrolysis of dilute H 2 SO 4, reduction occurs at the anode. the anode reaction is 2H + + 2e H 2 the anode reaction is 4OH O 2 + 4H + + 4e. the anode reaction is 2H 2 O 4H + + O 2 + 4e. 59. During the electrolysis of an aqueous solution of K 2 SO 4 the solution around the cathode becomes acidic. becomes more dilute. becomes basic. remains unchanged. (E) remains neutral 60. What product forms at the anode during the electrolysis of molten lithium iodide? Li + (l) Li(l) I - (l) I 2 (g) 7

8 61. Five metals are represented by the symbols L, M, T, R, and Z. When a solution containing all five ions at 1 M concentration is electrolyzed with a small applied voltage, which metal is most likely to be deposited first on the cathode? L L e 0.76 V R R e 0.34 V M M e 0.44 V T T e 0.13 V Z Z + + e 0.80 V L M T R (E) Z 62. The half-reaction that occurs at the cathode during the electrolysis of molten sodium bromide is: Br e - 2 Br - 2 Br - Br e - Na Na + + e - 2 H 2 O + 2 e - 2 OH - + H 2 (E) Na + + e - Na 63. Which requires the smallest voltage for electrolytic deposition given the reactivity order: Na > Al > Zn > Fe > Cu Na + Al 3+ Fe 2+ Zn 2+ (E) Cu How many coulombs of electricity are required to convert g of AgNO 3 into metallic Ag? A solution of CdSO 4 is electrolyzed between inert electrodes. How many hours must a current of 1.75 A flow to deposit 11.8 g of cadmium? Chromium metal can be plated from an acidic solution of CrO 3. How many grams of chromium will be plated by the passage of 19,300 C (coulombs)? What time is required to plate 2.08 g of copper from Cu 2+ at a constant current flow of 1.26 A? 41.8 min 128 min 83.6 min 5013 min 68. Nickel is a transition element and has a variable valence. Using a nickel salt, 2 F (faradays) plate out 39.2 g of nickel. What ions are in the solution of this salt? Ni + Ni 2+ Ni 3+ Ni 2/ A given amount of electric charge deposits g of silver from an Ag + solution. What mass of copper from a Cu 2+ solution will be deposited by the same quantity of electric charge? g 1.97 g 2.54 g 127 g 8

9 70. How many grams of aluminum are deposited from AlCl 3 in 30 min by a current of 40 A? 3.0 g 6.7 g 9.0 g 13.5 g (E) 27.0 g 71. One liter of solution contains 32.0 g of Cr(NO 3 ) 2. What current would be used to deposit all of the chromium in 5.0 h? 0.72 A 1.4 A 1.9 A 9.9 A 72. An aqueous copper(ii) sulfate solution is electrolyzed for 45 minutes using a 3.2 amp current. What mass of copper is produced? 5.6 g 0.95 g 1.9 g 2.8 g (E) 4.6 g 73. An aqueous solution of an unknown salt of ruthenium is electrolyzed by a current of 2.50 amperes passing for 50.0 minutes. Reduction of ruthenium ions deposits g of Ru metal at the cathode. What was the charge on the ruthenium ions in the solution? (E) Calculate the quantity of charge necessary to produce 10 liters of H 2 (g) at STP from the electrolysis of water. 8.6 x 10 4 C 1.7 x 10 4 C 5.3 x 10 4 C 4.8 x 10 4 C (E) 3.7 x 10 4 C 75. How many grams of Ca metal can be produced by the electrolysis of molten CaBr 2 using a current of 30.0 A for 10.0 hours? g 448 g 112 g 22.4 g (E) 224 g ANSWERS: A C C D C B C C A C D C B C D C D C D B C D C C A D C D B C D A C A A A A C D D D B A C A C D D B B B C A D C B E D C D E E E C C C C C A B C D D A E 9